Field of the Invention
The present invention relates generally to medical devices and methods. More particularly, the present invention relates to electrode structures and systems for delivering electrical pulses directly to the spinal cord of a patient to block pain and for other purposes.
The use of spinal cord stimulation (SCS) to relieve intractable pain symptoms originated in the 1960's along with emerging theories of the neural basis of pain perception and the pathophysiology of chronic pain disorders. Results from experimental animal studies demonstrated the existence of neural pathways that originate within the brain and project axons through the spinal cord that eventually terminate at spinal cord levels where pain signals from the peripheral nervous system enter the central nervous system. These pathways are postulated to play a role in the ‘top-down’ modulation of pain perception. Human SCS studies were initiated based on the theory that by using electrical stimulation to artificially activate descending pathways within the dorsal column of the spinal cord, the processing of pain related signals below the stimulation site could be attenuated, blocked or otherwise modulated.
Although the specific neural mechanisms that underlie the clinical efficacy of this treatment remain poorly understood, there is now abundant clinical evidence that SCS is capable of providing sustained pain relief to select patients with intractable chronic pain. The most important limitation of this treatment method is that a high percentage of patients implanted with an SCS system or device may experience only marginal improvement, or no improvement, in their pain symptoms. Treatment success rates of 50% or less are frequently reported with known SCS systems.
The neural mechanisms that mediate the clinical effects of SCS are complex and likely involve activation of multiple ascending and descending neural pathways within the spinal cord. Based on empiric clinical evidence, a number of treatment concepts have emerged to guide SCS strategies. In general, electrical stimulation will evoke sensory perceptions in the painful area of the body in order for the treatment to be effective. To accomplish this, the region within the dorsal column of the spinal cord that contains axons that are functionally related to the painful body area must be activated. Dorsal column axons are somatotopically organized, meaning that the axons that are functionally related to a particular body area are positioned in close proximity to each other, and there is an orderly anatomical pattern of organization within the spinal cord for the different groups of axons linked to different body areas. In the cervical spinal cord, for example, dorsal column axons functionally linked to the back region may be relatively close to the midline of the spinal cord, and axons linked to the arms are positioned relatively more laterally.
Adverse effects of electrical stimulation can result from unintended activation of non-targeted neural structures. When the dorsal nerve rootlets are activated, for example, discomfort can result. The effectiveness of SCS treatment is generally dependent on the capacity of the device to selectively activate targeted axons within a specific sub-region of the dorsal column, without activating the nearby dorsal rootlets. This concept is incorporated into researchers use of the term therapeutic range to describe the range of stimulus intensities that are above perceptual threshold (i.e. effectiveness threshold) but below the discomfort threshold, beyond which stimulation effects are no longer tolerated by the patient. The ideal SCS device will be capable of efficiently and safely delivering highly focused electrical stimuli to the targeted sub-region of the dorsal column without activating nearby structures. The electrode contact should be positioned as close to the targeted-axons as possible and the resulting volumetric pattern of tissue activation should tightly conform to the anatomy of the targeted neural pathway.
The spinal cord is cylindrically shaped and positioned centrally within the spinal canal. The spinal canal is lined by a dural membrane and contains cerebrospinal fluid (CSF) that surrounds the spinal cord and fills the region between the outside surface of the spinal cord and the inside surface of the dural membrane. This CSF-filled space plays a critical role in normal spinal cord biomechanics and is an important factor that should be considered when performing spinal surgery. During normal movements, such as flexion and extension of the body, the spinal cord moves within the spinal canal, altering its position relative to the dural lining of the spinal canal. The volume of CSF surrounding the spinal cord serves as a frictionless buffer during these movements. In some pathological conditions (e.g. tethered cord syndrome) this normal motion of the spinal cord is impeded by tissue attachments bridging the space between the spinal cord and the dural lining, resulting in dysfunction of the spinal cord. In other pathological conditions, a tissue barrier forms within the spinal canal (e.g. following trauma or infection) that disrupts the normal flow of CSF over the surface of the spinal cord. In this setting CSF may accumulate within the substance of the spinal cord to form a syrinx and cause neurological dysfunction.
The dural listing of the spinal canal should be managed with particular care during spinal surgery. If a detect is created in this lining, a CSF fistula may develop which increases the risk of a wound complication (infection or dehiscence) and may cause the patient to experience disabling positional headaches. In order to access the spinal cord itself, the dural membrane should be opened surgically and this is performed in a manner that allows the surgeon to achieve a ‘water-tight’ closure at the completion of the operation. Typically this involves sharply incising the dura over the dorsal aspect of the spinal canal, a location that is readily accessible and well visualized during surgery. Later the dura is re-approximated by suturing together the well defined cut margins of the fibrous membrane. This closure technique is performed in a manner that preserves the CSF filled space separating the dura from the spinal cord, thus preventing mechanical constriction, or tethering, at the surgical site.
These anatomical and surgical considerations have impacted the evolution of a wide range of operative procedures, including spinal cord stimulator surgery. When the design intent is to minimize the risk of surgical complications, the optimal strategy is to entirely avoid opening the dural membrane and place the implant outside of the dura (extra-dural procedure). If the spinal cord must be accessed directly (intra-dural procedure) the operation should be designed in a manner that prevents CSF fistula formation, mechanical tethering of the spinal cord to the dura, or physical obstruction of the CSF filled space surrounding the spinal cord.
There are limitations in the performance characteristics of the prior art. One such limitation is the following. Existing SCS devices deliver electrical stimuli through electrodes placed outside of the fibrous lining of the spinal canal (dura). This results in inefficient and poorly localized patterns of spinal cord activation due to the electrical shunting effect of cerebrospinal fluid that fills the space separating the dural lining and the spinal cord. This inability to selectively activate targeted regions of the spinal cord is thought to be an important contributing factor to the significant incidence of sub-optimal or poor treatment outcomes with existing SCS devices. Despite these limitations large numbers of patients are implanted. The size of the SCS market attests to the large scope of this public health problem and the fact that under certain circumstances, electrical activation of the spinal cord provides pain relief for patients who have failed all other treatment modalities.
A further limitation of the prior art arises in the nature of certain tethered forms of spinal cord stimulators. When SCS electrodes were first placed in human subjects, most were implanted on the surface of the dura, but in some instances the dura was opened and electrodes were placed directly on the surface (intradural) of the spinal cord (Gildenberg 2006, Long 1977, Long 1998, Shealy et al. 1970). The wires from electrodes placed directly on the spinal cord passed through the dura, thus mechanically tethering the electrode to the dura. The electrodes were constructed of conventional conductive and insulting materials, were bulky, and had a limited number of contacts through which stimuli could be delivered. The locations of the contacts relative to targeted and near-targeted neural structures were difficult to control and could not be adjusted following the implantation surgery. Because of these factors, and the increased risks associated with opening the dura, at the time there was no obvious therapeutic advantage to the intradural approach. The use of intradural stimulating electrodes was eventually discontinued and currently all SCS devices use extradural stimulating electrodes.
Still another limitation of the prior art arises in terms of the treatment efficacy. There are two broad classes of extradural stimulation electrodes. One type can be placed percutaneously through a needle into the epidural space. These electrodes have small cylindrically shaped contacts positioned along the shaft of a flexible linear electrode array. They are used either for minimally invasive testing of stimulation effects prior to implantation surgery, or as the device that is permanently implanted. The other type of extradural electrode is placed during an open surgical procedure and consists of a flat array of multiple electrode contacts positioned over the exposed dural surface. An experienced practitioner is capable of implanting these extradural electrodes with a high degree of safety. However, the current SCS devices have suboptimal treatment efficacy. We hypothesize that this shortcoming is due in large part to the inability of extradural electrodes to selectively activate the targeted sub-region of the dorsal column of the spinal cord. By placing devices outside of the dura because of safety considerations, an intrinsic disadvantage is incurred in terms of therapeutic efficacy. The presence of a CSF filled space between an extradural stimulating electrode and the spinal cord profoundly degrades the ability of the device to create a volume of electrical activation that selectively encompasses the targeted sub-region of the spinal cord. This results from the conductive properties of CSF. CSF is a far more efficient electrical conductor than any other tissue in the spine (Holsheimer 1998). When an electrical stimulus delivered by an extradural electrode traverses the dura and enters the CSF-filled space between the dura and the spinal cord, a large fraction of the stimulus is electrically ‘shunted’ diffusely within this CSF filled space. Researchers estimate that extradural stimulation results in the spinal cord receiving less than 10% of the delivered stimulus. The stimulus effect penetrates the spinal cord to a distance of 0.25 mm or less and the broad volumetric pattern encompasses both targeted (i.e. dorsal column) and non-targeted (i.e. dorsal rootlets) neural structures (He et al. 1994, Holsheimer 1998, Holsheimer 2002, Holsheimer et al. 2007).
The clinical importance of these limitations of the prior art are reflected in the numerous efforts made by device manufactures to mitigate the problems. These include the development of spatially distributed multi-contact extradural arrays and stimulation protocols that enable delivery of electrical charge distributions over widely variable anatomical patterns. This strategy allows the physician to adjust the anatomical location of maximal stimulation on the dural surface, but the presence of CSF shunting continues to markedly attenuate the stimulation effects within the spinal cord. Clinicians have also used a strategy of placing multiple cylindrical electrodes within the extradural space tor the purpose of mechanically reducing the size of the CSF-filled space and displacing the electrode contacts to a position closer to the spinal cord (Holsheimer et al. 2007). A device modification recently introduced by one of the largest manufacturer of SCS devices seeks to address problems associated with movement of the spinal cord within the CSF-filled spinal canal that occurs when patients change position. These positional changes after the spatial relationship between an extradural electrical source and the spinal cord, and the pattern of tissue activation. The new device senses patient position and automatically adjusts stimulus parameters for the purpose of achieving stable therapeutic effects. As with all other SCS design changes introduced to-date, the addition of a position sensor does not address the fundamental problem of CSF shunting of the electrical stimulus.